Trauma refers to injuries caused by an impact on a body even in the absence of penetration. For example, broken bones, internal bleeding and shock commonly result from shooting incidents, even when the bullet is stopped by a bullet-proof vest or other protective garment incorporating ballistic resistant fabric. In addition to injuries resulting after a ballistic projectile has been stopped by a bullet resistant vest, trauma can be caused through any substantial impact force. Examples of such impacts include sports injuries such as those sustained in football, baseball, and cycling accidents. Other area of common occurrence injuries would include falls on stairs, automobile crashes and industrial accidents as well as massive collision injuries such as those sustained in survivable airplane crashes.
Ballistic resistant fabrics, sometimes referred to as bullet-proof materials, serve to protect against penetration by a bullet or other object. There are two major markets for ballistic resistant fabrics—military and police. A third potential market, civilian applications, is presently limited to the executive business and political community by manufacturers of such garments so criminals cannot purchase these items to use in crimes. Stopping a projectile prior to entry into the body, however, does not mean that a person will necessarily survive its impact. National Institute of Justice standards differentiate ballistic protection between handguns and rifles and take into account trauma damage by measuring the deflection of the target into Roma Plastilina Number One clay backing. A deflection of 44 mm or less is considered adequate in the test. Although no correlation between this test and human subjects has been officially established, it is known that the reduction of trauma increases the likelihood of survival and reduces recovery time and medical costs. NIJ has not documented injury effects related to trauma sustained after bullets were stopped.
An important element of survival, whether it is the survival of penetration by a projectile or the impact trauma from a projectile, is the dissipation of energy prior to the projectile reaching the body. The dissipation of impact energy by a material is a measure of the efficiency of the energy absorption mechanism. The fiber response to a projectile impact is presently understood to involve elongation, slippage and breakage. Strain or compression wave velocity is expressed as v=√F/μ, where v=strain wave velocity, F=impact force and μ=linear density expressed as kg/m. At the same time, one can also express v as √E/ρ, where E=Young's modulus and ρ=specific gravity of material. The expression F=Eμ/ρ indicates optimum dissipation of impact energy. Structures that optimize each of these properties yield the best ballistic performance.
Woven fabric dissipates energy, in this case energy transferred by impact from a projectile, at the yarn interlacings of the fabric. Thus, the energy must be distributed along the yarn axis to each interlacing point for dissipation. As a result, woven fabrics are believed to lose about one third of their strength as a result of weaving, with additional loss resulting from mechanical interaction between warp and weft yarns during tensile loading. High warp crimp in a woven structure is typically accompanied by low strength translation efficiency.
Non-woven materials typically do not suffer from this disadvantage. Manufacture of non-wovens by needlepunching is a simple operation by which a variety of properties can be obtained in the fabric by varying elements of the process in known manner, and at a substantial cost reduction over woven materials. A 1966 U.S. Department of Defense study found that a needlepunched structure containing ballistic resistant nylon could be produced at one-third the weight of a woven duck fabric while retaining 80% of its ballistic resistance. Non-wovens are currently being used in special applications such as DSM designed “Fraglight” with Dyneema® fibers to stop fragments.
As polymer science has progressed, “ballistic” fibers such as high tenacity polyamides, aramids and linear high-density polyethylene (HDPE) have been developed, and have been found to be applicable for ballistic resistant applications. The protection offered per unit weight of the material has increased greatly along with greater comfort and less bulk.
Despite such improvements in ballistic-resistant materials for preventing penetration by a projectile, developments in the reduction of non-penetration trauma have not advanced as rapidly. Known trauma reduction methods include the provision of rigid plates and polyurethane foams. Plates used for trauma reduction are generally heavy and uncomfortable, and are not permeable to air or moisture such that a garment can breathe. And some plates have been found to break or deform upon high-energy impact, sometimes causing the plate to become a projectile capable of inflicting injury. Known foam padding is typically uncomfortably thick, and also traps heat and moisture.
Thus, it can be seen that needs exist for improved materials for absorbing energy from impact by or with an object and reducing resultant trauma to the body, and for related methods and devices.